ITS 



THE AMERICAN NATURALIST [Vol. XLIII 



the gametes and their subsequent division, a decrease 

 in this stimulation in species which have for ages de- 

 pended upon cross-fertilization may result in decreased 

 fertility. 



We can scarcely form a definite idea of the mechanism 

 through which such a stimulation may take place. There 

 may. be chemical compounds found in different strains 

 that react when brought together. If this were the case 

 we should expect to find separate families with like 

 " compounds" which when crossed would not be more 

 vigorous than when inbred. It would be difficult to estab- 

 lish such a thing experimentally. On the other hand, 

 the actual fact of difference 5 in gametic constitution may 

 set up a biological "action" analogous to ionization. If 

 this were true, and individuals of the F x generation 

 heterozygous in all differentiating characters were 

 selected in succeeding generations, there should be no 

 reduction in vigor, while individuals of the F 2 genera- 

 tion, homozygous in their characters, should compare in 

 vigor to the P x generation. This hypothesis we are test- 

 ing, but results will necessarily be slow. 



The F x generation of thirty maize crosses were grown 

 in 1908 on well fertilized land in Connecticut. They 

 were planted three feet six inches each way, about four 

 stalks to the hill. Seeds from the same parent ears 6 

 which were used to make the crosses were also grown 

 for comparison. Only fifty hills of each of the crosses 

 and of each parent could be grown on account of limited 

 space, but the soil conditions were such that a very fair 



8 The objection will be raised that beyond a certain amount of difference 

 between gametes, there will be sterility. It is generally true that there is 

 sterility with wide differences in botanical or zoological characters, but 

 there are exceptions, and we must not fall into the same old rut of putting 

 the two down as cause and effect because there is at present no other 

 explanation. It is definitely settled that in certain cases the bar to fertility 



ovum. It seems reasonable that the stimulation effect may be illustrated by 

 borrowing Galton's polyhedron. 



• The parent ears were, therefore, one year older, but their germination 

 was good, and their growth equal to inbred seed of the same ages as the 

 hybrid seed. 



